Fixed #31295 -- Avoided Select widget triggering additional query when using ModelChoiceIterator.fix-31295

5 files changed, 51 insertions, 9 deletions

diff --git a/django/forms/models.py b/django/forms/models.py
index 9686baa6f2..c04a806caa 100644
--- a/django/forms/models.py
+++ b/django/forms/models.py
@@ -3,6 +3,7 @@ Helper functions for creating Form classes from Django models
 and database field objects.
 """
 
+from collections import deque
 from itertools import chain
 
 from django.core.exceptions import (
@@ -1438,17 +1439,20 @@ class ModelChoiceIterator(BaseChoiceIterator):
     def __init__(self, field):
         self.field = field
         self.queryset = field.queryset
+        self._deque = deque()
+        self._generator = self.generator()
 
-    def __iter__(self):
+    def generator(self):
         if self.field.empty_label is not None:
             yield ("", self.field.empty_label)
-        queryset = self.queryset
-        # Can't use iterator() when queryset uses prefetch_related()
-        if not queryset._prefetch_related_lookups:
-            queryset = queryset.iterator()
-        for obj in queryset:
+        for obj in self.queryset.iterator(chunk_size=100):
             yield self.choice(obj)
 
+    def __iter__(self):
+        yield from chain(self._deque, self._generator)
+        self._deque = deque()
+        self._generator = self.generator()
+
     def __len__(self):
         # count() adds a query but uses less memory since the QuerySet results
         # won't be cached. In most cases, the choices will only be iterated on,
@@ -1456,7 +1460,9 @@ class ModelChoiceIterator(BaseChoiceIterator):
         return self.queryset.count() + (1 if self.field.empty_label is not None else 0)
 
     def __bool__(self):
-        return self.field.empty_label is not None or self.queryset.exists()
+        return (
+            self.field.empty_label is not None or self._deque or self.queryset.exists()
+        )
 
     def choice(self, obj):
         return (
@@ -1464,6 +1470,12 @@ class ModelChoiceIterator(BaseChoiceIterator):
             self.field.label_from_instance(obj),
         )
 
+    def peek(self):
+        value = next(self._generator, None)
+        if value is not None:
+            self._deque.append(value)
+        return value
+
 
 class ModelChoiceField(ChoiceField):
     """A ChoiceField whose choices are a model QuerySet."""
diff --git a/django/forms/widgets.py b/django/forms/widgets.py
index 1bcfeba288..82685a3fea 100644
--- a/django/forms/widgets.py
+++ b/django/forms/widgets.py
@@ -851,7 +851,11 @@ class Select(ChoiceWidget):
         if self.allow_multiple_selected:
             return use_required_attribute
 
-        first_choice = next(iter(self.choices), None)
+        first_choice = (
+            self.choices.peek()
+            if hasattr(self.choices, "peek")
+            else next(iter(self.choices), None)
+        )
         return (
             use_required_attribute
             and first_choice is not None
diff --git a/docs/ref/forms/fields.txt b/docs/ref/forms/fields.txt
index 8ddc5b9d79..6186f7d4e8 100644
--- a/docs/ref/forms/fields.txt
+++ b/docs/ref/forms/fields.txt
@@ -1619,12 +1619,16 @@ customize the yielded 2-tuple choices.
 
     ``ModelChoiceIterator`` has the following method:
 
-    .. method:: __iter__()
+    .. method:: generator()
 
         Yields 2-tuple choices, in the ``(value, label)`` format used by
         :attr:`ChoiceField.choices`. The first ``value`` element is a
         :class:`ModelChoiceIteratorValue` instance.
 
+    .. versionchanged:: 6.1
+
+        ``generator`` yields choices instead of ``__iter__``.
+
 ``ModelChoiceIteratorValue``
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/docs/releases/6.1.txt b/docs/releases/6.1.txt
index 987d46874a..cc236b5ac2 100644
--- a/docs/releases/6.1.txt
+++ b/docs/releases/6.1.txt
@@ -264,6 +264,12 @@ Forms
   field's choices. This allows per-request refreshing when called in a form's
   ``__init__()``.
 
+* :class:`~django.forms.ModelChoiceIterator` now yields choices via
+  its :meth:`~django.forms.ModelChoiceIterator.generator` method.
+  Users who have previously overridden the ``__iter__()`` method of
+  :class:`~django.forms.ModelChoiceIterator` should override
+  :meth:`~django.forms.ModelChoiceIterator.generator` instead.
+
 Generic Views
 ~~~~~~~~~~~~~
 
diff --git a/tests/model_forms/test_modelchoicefield.py b/tests/model_forms/test_modelchoicefield.py
index 40c625da7c..387caf0778 100644
--- a/tests/model_forms/test_modelchoicefield.py
+++ b/tests/model_forms/test_modelchoicefield.py
@@ -169,6 +169,22 @@ class ModelChoiceFieldTests(TestCase):
         Category.objects.all().delete()
         self.assertIs(bool(f.choices), False)
 
+    def test_empty_label_one_query(self):
+        class MyForm(forms.Form):
+            f = forms.ModelChoiceField(Category.objects.all(), empty_label=None)
+
+        form = MyForm()
+        with self.assertNumQueries(1):
+            form.as_ul()
+        # Ensure the query re-runs when re-rendering the form.
+        with self.assertNumQueries(1):
+            form.as_ul()
+
+        # Ensure the iterator stops after yielding its choices.
+        choice_iterator = iter(form.fields["f"].choices)
+        self.assertEqual(len(list(choice_iterator)), 3)
+        self.assertEqual(len(list(choice_iterator)), 0)
+
     def test_choices_bool_empty_label(self):
         f = forms.ModelChoiceField(Category.objects.all(), empty_label="--------")
         Category.objects.all().delete()